Subject: [PATCH v24 5/9] arm64: kdump: add kdump support

From: [email-masked] (AKASHI Takahiro)
Date: Thu, 18 Aug 2016 16:15:48 +0900

Hi James, Pratyush,

Thank you for your testing and reporting an issue.
I've been on vacation until yesterday.

On Wed, Aug 10, 2016 at 05:38:05PM +0100, James Morse wrote:
> Hi Akashi,
> 
> On 09/08/16 02:56, AKASHI Takahiro wrote:
> > On crash dump kernel, all the information about primary kernel's system
> > memory (core image) is available in elf core header.
> > The primary kernel will set aside this header with reserve_elfcorehdr()
> > at boot time and inform crash dump kernel of its location via a new
> > device-tree property, "linux,elfcorehdr".
> > 
> > Please note that all other architectures use traditional "elfcorehdr="
> > kernel parameter for this purpose.
> > 
> > Then crash dump kernel will access the primary kernel's memory with
> > copy_oldmem_page(), which reads one page by ioremap'ing it since it does
> > not reside in linear mapping on crash dump kernel.
> > 
> > We also need our own elfcorehdr_read() here since the header is placed
> > within crash dump kernel's usable memory.
> 
> On Seattle when I panic and boot the kdump kernel, I am unable to read the
> /proc/vmcore file. Instead I get:
> nanook at frikadeller:~$ sudo cp /proc/vmcore /
> [  174.393875] Unhandled fault: synchronous external abort (0x96000210) at
> 0xffffff80096b6000
> [  174.402158] Internal error: : 96000210 [#1] PREEMPT SMP
> [  174.407370] Modules linked in:
> [  174.410417] CPU: 6 PID: 2059 Comm: cp Tainted: G S      W I     4.8.0-rc1+ #4708
> [  174.417799] Hardware name: AMD Overdrive/Supercharger/Default string, BIOS
> ROD1002C 04/08/2016
> [  174.426396] task: ffffffc0fdec5780 task.stack: ffffffc0f34bc000
> [  174.432313] PC is at __arch_copy_to_user+0x180/0x280
> [  174.437274] LR is at copy_oldmem_page+0xac/0xf0
> [  174.441791] pc : [<ffffff800835e080>] lr : [<ffffff8008095b9c>] pstate: 20000145
> [  174.449173] sp : ffffffc0f34bfc90
> [  174.452474] x29: ffffffc0f34bfc90 x28: 0000000000000000
> [  174.457776] x27: 0000000008000000 x26: 000000000000d000
> [  174.463077] x25: 0000000000000001 x24: ffffff8008eb5000
> [  174.468378] x23: 0000000000000000 x22: ffffff80096b6000
> [  174.473679] x21: 0000000000000001 x20: 0000000030127000
> [  174.478979] x19: 0000000000001000 x18: 0000007ff7085d60
> [  174.484279] x17: 0000000000429358 x16: ffffff80081d9e88
> [  174.489579] x15: 0000007fae377590 x14: 0000000000000000
> [  174.494880] x13: 0000000000000000 x12: ffffff8008dd1000
> [  174.500180] x11: ffffff80096b6fff x10: ffffff80096b6fff
> [  174.505480] x9 : 0000000040000000 x8 : ffffff8008db6000
> [  174.510781] x7 : ffffff80096b7000 x6 : 0000000030127000
> [  174.516082] x5 : 0000000030128000 x4 : 0000000000000000
> [  174.521382] x3 : 00e8000000000713 x2 : 0000000000000f80
> [  174.526682] x1 : ffffff80096b6000 x0 : 0000000030127000
> [  174.531982]
> [  174.533461] Process cp (pid: 2059, stack limit = 0xffffffc0f34bc020)
> 
> [  174.848448] [<ffffff800835e080>] __arch_copy_to_user+0x180/0x280
> [  174.854448] [<ffffff8008245f34>] read_from_oldmem.part.4+0xb4/0xf4
> [  174.860615] [<ffffff8008246074>] read_vmcore+0x100/0x22c
> [  174.865919] [<ffffff8008239378>] proc_reg_read+0x64/0x90
> [  174.871223] [<ffffff80081d7da8>] __vfs_read+0x28/0x108
> [  174.876348] [<ffffff80081d8ae4>] vfs_read+0x84/0x144
> [  174.881301] [<ffffff80081d9ecc>] SyS_read+0x44/0xa0
> [  174.886167] [<ffffff8008082ef0>] el0_svc_naked+0x24/0x28
> [  174.891466] Code: 00000000 00000000 00000000 00000000 (a8c12027)
> [  174.897562] ---[ end trace 00801b2e35b0cd1f ]---
> 
> 
> The offending call is:
> > copy_oldmem_page(0x8000000, 0x00000000385f8000, 0x1000, 0, 1)
> 
> This is trying to access the bottom page of memory. From the efi memory map:
> > efi:   0x008000000000-0x008001e7ffff [Runtime Data       |RUN|  |WB|WT|WC|UC]*
> > efi:   0x008001e80000-0x008001ffffff [Conventional Memory|   |  |WB|WT|WC|UC]
> 
> This page is 'Runtime Data', and marked as nomap by both the original and kdump
> kernels, but copy_oldmem_page() doesn't know this.
> 
> In this case because we have already parsed the efi memory map again in the
> kdump kernel and re-marked these regions as nomap, the below hunk fixes the
> problem for me:
> =========================%<=========================
> diff --git a/arch/arm64/kernel/crash_dump.c b/arch/arm64/kernel/crash_dump.c
> index 2dc54d129be1..784d4c30b534 100644
> --- a/arch/arm64/kernel/crash_dump.c
> +++ b/arch/arm64/kernel/crash_dump.c
> @@ -37,6 +37,11 @@ ssize_t copy_oldmem_page(unsigned long pfn, char *buf,
>         if (!csize)
>                 return 0;
> 
> +       if (memblock_is_memory(pfn << PAGE_SHIFT) &&
> +           !memblock_is_map_memory(pfn << PAGE_SHIFT))
> +               /* skip this nomap memory region, reserved by firmware */
> +               return 0;
> +
>         vaddr = ioremap_cache(__pfn_to_phys(pfn), PAGE_SIZE);

Here I'm wandering why my original code doesn't work.
If !memblock_is_map_memory(), ioremap_cache() would call __ioremap_caller()
and return a valid virtual address mapped in vmalloc area.

>         if (!vaddr)
>                 return -ENOMEM;
> =========================%<=========================
> 
> With this I can copy the vmcore file, and feed it to crash to read dmesg, task
> list etc...
> 
> This could be a deeper/wider issue, but I can't see any other users of
> memblock_mark_nomap().
> Do you think depending on this this 're-learning' is robust enough, or should
> the nomap ranges be described in the vmcoreinfo elf notes?

The current kexec-tools identifies all the memory regions from
/proc/iomem and there is no way for user space tools to distinguish
"EFI runtime data," or any other nomap memory, from normal "System RAM"
because all those resources are currently marked as "System RAM."

So I think that such regions should be marked as, say, "reserved,"
so that we can exclude those memories from a crush dump file.

(I don't know whether this change may have a backward-compatibility
problem.)

-Takahiro AKASHI

> 
> Thanks,
> 
> James
> 
> 
> > diff --git a/arch/arm64/kernel/crash_dump.c b/arch/arm64/kernel/crash_dump.c
> > new file mode 100644
> > index 0000000..2dc54d1
> > --- /dev/null
> > +++ b/arch/arm64/kernel/crash_dump.c
> > @@ -0,0 +1,71 @@
> > +/*
> > + * Routines for doing kexec-based kdump
> > + *
> > + * Copyright (C) 2014 Linaro Limited
> > + * Author: AKASHI Takahiro <[email-masked]>
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License version 2 as
> > + * published by the Free Software Foundation.
> > + */
> > +
> > +#include <linux/crash_dump.h>
> > +#include <linux/errno.h>
> > +#include <linux/io.h>
> > +#include <linux/memblock.h>
> > +#include <linux/uaccess.h>
> > +#include <asm/memory.h>
> > +
> > +/**
> > + * copy_oldmem_page() - copy one page from old kernel memory
> > + * @pfn: page frame number to be copied
> > + * @buf: buffer where the copied page is placed
> > + * @csize: number of bytes to copy
> > + * @offset: offset in bytes into the page
> > + * @userbuf: if set, @buf is in a user address space
> > + *
> > + * This function copies one page from old kernel memory into buffer pointed by
> > + * @buf. If @buf is in userspace, set @userbuf to %1. Returns number of bytes
> > + * copied or negative error in case of failure.
> > + */
> > +ssize_t copy_oldmem_page(unsigned long pfn, char *buf,
> > +			 size_t csize, unsigned long offset,
> > +			 int userbuf)
> > +{
> > +	void *vaddr;
> > +
> > +	if (!csize)
> > +		return 0;
> > +
> > +	vaddr = ioremap_cache(__pfn_to_phys(pfn), PAGE_SIZE);
> > +	if (!vaddr)
> > +		return -ENOMEM;
> > +
> > +	if (userbuf) {
> > +		if (copy_to_user(buf, vaddr + offset, csize)) {
> > +			iounmap(vaddr);
> > +			return -EFAULT;
> > +		}
> > +	} else {
> > +		memcpy(buf, vaddr + offset, csize);
> > +	}
> > +
> > +	iounmap(vaddr);
> > +
> > +	return csize;
> > +}
>